1. Technical Field
The technical field relates to a driving circuit for light-emitting diodes (LEDs), and especially to a driving circuit used for LEDs and adapted to input voltages of various magnitudes.
2. Description of Related Art
In LED lighting system, driving circuit is crucial account for the performance and the cost of overall system, therefore, the design of driving circuit is important issue for the LED lighting system.
Linear driver is one of major driving schemes for LED lighting system and has advantages of simple design and immunity to electromagnetic interference (EMI).
However, the current linear driver for LED lighting system has demanding requirement for matching between input and output voltages. Namely, the linear driver fails to work when the overall forward voltage of the LED is higher than the input voltage. On the contrary, the transistor switch (generally MOSFET switch or BJT switch) in series with the LED will have large voltage stress when the overall forward voltage of the LED is excessively lower than the input voltage.
Preferably, the overall forward voltage of the LED is slightly lower than the input voltage for the LED lighting system worked in high AC input voltages such as 200˜240 volts, whereby the LED lighting system can work normally and the voltage stress for the MOSFET switch or BJT switch can be reduced. Nevertheless, the above high AC-input LED lighting system cannot work in low AC-input application (such as 100˜120 volts) because the input voltage is generally lower than the overall forward voltage of the LED.
FIG. 1 shows a related art linear driver for LED lighting system, a bridge rectifier 10A rectifies an AC input Vac into an input voltage Vin. The LED strings 21A and 22A are electrically connected to constant-current circuits 31A and 32A, respectively and the constant-current circuits 31A and 32A control the current flowing through the LED strings 21A and 22A via corresponding transistor switches Q1A and Q2A in order to drive the LED strings 21A and 22A as pure resistive load.
However, in above-mentioned related art linear driver for LED lighting system, considerable voltage drop is across the transistor switches Q1A and Q2A in the constant-current circuits 31A and 32A when the input voltage is much higher than the driving voltage VLED of the LED strings 21A and 22A. The power dissipation associated with the voltage drop VQ is absorbed by the transistor switches Q1A and Q2A. The transistor switches Q1A and Q2A has risk of damage when the power dissipation thereof is excessively high. On the contrary, the LED strings 21A and 22A cannot be turned on when the input voltage Vac is lower than the driving voltage VLED of the LED strings 21A and 22A
The commercially available LED lamps are generally classified in terms of different input voltages corresponding to different countries/regions such that the LED lamp can satisfy rated voltage in the selling countries/regions. Namely, the LED lamp with single specification cannot be used for worldwide voltage. It is inconvenient for user to spend extra cost to buy lamp suitable for local region. For circuit designer, more labor and cost are needed to develop LED lamps adapted for the various voltages of different regions.
Therefore, it is desirable to provide a driving circuit for LED, which can maintain constant current and voltage operation for the LED module and meet the input/output matching requirement for linear driver as well as render the LED applicable to worldwide voltage.